It has been appreciated that an aluminum-silicon interaction occurs when aluminum is in direct contact with silicon at elevated temperatures. W. Chu et al in IBM Technical Disclosure Bulletin, Volume 19, No. 7, p. 2532 (December 1976), suggest employing a transition metal oxide as an intermediate layer between the aluminum contact and the silicon to overcome this problem. While the Chu et al technique eliminates interaction of aluminum with silicon, a high resistance contact may result due to the existence of the interface between the transition metal oxide and the silicon.
An alternative solution to the use of transition metal oxides as a reaction barrier has been to interpose silicides and/or metallic layers between the underlying silicon and the conductor metallurgy.
When a silicide is interposed between an aluminum conductor and a silicon substrate, then, depending on the impurity concentration in the silicon either an ohmic or Schottky barrier contact as well as a reaction barrier is formed between the aluminum and the silicon. However, the aluminum conductor may react with the silicide during subsequent elevated temperature processing steps such as annealing, and result in penetration of the aluminum through the silicide. This can result in alteration of the electrical characteristic of the original silicide-silicon contact, e.g. in a Schottky barrier and it may result in electrically shorting the height of the underlying junction. This problem has been addressed by Hollins in U.S. Pat. No. 3,906,540 and is solved by using an intermediate layer of a refractory metal, such as Mo, Ti, W, Ta, or alloys thereof, to block the interaction of the aluminum with the silicide. The use of Cr as the intermediate layer between an aluminum contact and a silicide was pointed out by T. M. Reith and M. Revitz in IBM Technical Disclosure Bulletin, Vol. 16, No. 11, page 3586 (April 1974). Since the silicide is usually formed by depositing a metal onto the silicon substrate, and thereafter heating to react and form a silicide, the problem of a high resistance interface does not exist between the silicide and the silicon substrate. Instead a highly resistive interface can exist between the refractory metal and the silicide because of the nature of such an interface, and also, because of possible contamination of the silicide refractory metal interface. In addition, generation of this hierarchy of metallic film composites can lead to unwanted stress phenomena, embrittlement, or delamination which impairs the reliability of the contact.
P. S. Ho et al in the IBM Technical Disclosure Bulletin, Vol. 21, No. 8, page 3372, January 1979, have extended the principle of using an intermediate layer between an aluminum contact and the silicide layer. Their technique substitutes either aluminum-palladium or aluminum-platinum alloys for the refractory metals. These alloys serve a function similar to that served by the refractory metal by providing a barrier to prevent the interaction of the aluminum with the silicide.
The methods of Ho et al, Reith et al, and Hollins, all require a silicide to be formed on a silicon substrate before their method can be practiced. As has been mentioned earlier, the silicide is usually formed by depositing a metal onto the silicon substrate and thereafter heating to react with the silicon substrate to form a silicide. Thus, in general, the formation of the silicide, such as PtSi and Pd.sub.2 Si, results in consumption of an amount of silicon from the substrate about equal to the amount of metal deposited thereon. In the case of shallow junctions which are to be contacted such as those in small insulated gate field effect transistors (FET's), the consumption of silicon in forming a useable contact can result in depletion of a significant portion, of the active region of the device making the design of a practical process which takes into account process tolerances very difficult. When extremely shallow junctions are to be contacted there may be no practical process.
Alternatively, Rosvold in U.S. Pat. No. 3,938,243 teaches the co-deposition of Pt and Ni onto a silicon substrate and thereafter reacting to form a ternary alloy with silicon. The resulting ternary alloys which he describes are mixtures of about equal amounts of Pt-Ni and silicon. These alloys will interact with an aluminum conductor in a fashion similar to the reaction of other silicides unless methods such as those of Ho et al, Reith et al, or Hollins are employed to prevent such interaction. Furthermore, formation of such ternary silicides consume silicon in an amount similar to what occurs during the formation of PtSi for Pd.sub.2 Si making these ternary alloys marginally useful for shallow junctions.
Crowder et al in a copending application, Ser. No. 811,914 assigned to the assignee of the present application, discloses a method for codepositing a silicide onto a silicon substrate by co-evaporation of silicon and a silicide forming metal. While this technique eliminates the problems of the consumption of silicon from the substrate it does not offer an opportunity for reaction of the deposited silicide with the substrate. Since the deposited layer does not react with the substrate, a high resistance interface is probably created between the substrate and the deposited material, thereby limiting its effectiveness as a contact
Furthermore, this technique does not address the problem of the Al-silicide penetration phenomenon, and further processing steps, such as those suggested by Ho et al, and Hollins, will be required to overcome this problem.
The use of an intermetallic compound as a reaction barrier has been proposed by Magdo in U.S. Pat. No. 3,995,301 assigned to the present assignee. This patent teaches depositing aluminum onto a silicide layer, thereafter heat treating in a temperature range of 400.degree. or 450.degree. to transform the silicide to form an aluminum-platinum compound of the form of Al.sub.2 Pt. This technique provides a new contact of Al.sub.2 Pt to the silicon substrate which lowers the barrier height from about 0.8 eV of PtSi to about 0.72 eV. The Magdo patent requires the deposition of a silicide forming metal onto the silicon substrate and reacting thereafter to form a silicide. Since a silicide layer must be formed during processing this technique will be subject to the same limitations as earlier set forth with respect to the formation of a silicide barrier contact on shallow junction devices. Using a similar approach, and Howard et al in U.S. Pat. No. 4,140,020 discloses and claims the formation of binary intermetallic compounds on the surfaces of silicon substrates used for contacts and Schottky barriers. In this latter technique, metals forming binary intermetallic compounds are codeposited and subsequently reacted by heat treating. Again, this technique suffers from the same shortcoming as does the method of Crowder et al in that there will be no reaction between the silicon substrate and the layers deposited thereon which is necessary to avoid formation of a high resistance interface.